He said NYMEX had no set timetable to launch the emissions.

The global carbon market last year tripled to US$30 billion, with the lion's share taking place on the European Union's emissions market, according to the World Bank. The EU launched the market in 2005 to help countries meet their emissions obligations under the Kyoto Protocol on global warming.

The United States is the world's top emitter of greenhouse gases, such as carbon dioxide, but does not regulate them. President George W. Bush withdrew the country from the Kyoto pact in 2001.

Over the last few years, quiet US business-to-business voluntary emissions trades and a "retail" emissions market in the United States have developed in the absence of federal regulation. Levin said those were "great experiments, but certainly there is a reservoir of criticism on some of those programs and we are trying to take all of that into account as we evaluate it."

Several bills in the US Congress would set up a cap and trade market in which companies that cut emissions under a set target could sell credits representing the reductions to companies that did not cut emissions.

Voluntary emissions allowances trade on the Chicago Climate Exchange among companies and local governments that commit to reducing output of the gases. The bourse is not open to financial speculators, however, which is one of the strengths of the NYMEX.

Levin said the idea of trading credits for the right to emit greenhouse gases "sounds simple on paper" but that there is much to mull before offering any products.

Levin said the NYMEX could consider trading voluntary or mandatory allowances. The NYMEX would consider trading offsets, in which an entity could earn emissions credits for investing in a project that deemed to cut emissions, such as a wind or solar power project, or burning methane at farm manure lagoons or escaping from coal mines.